1. Field of the Invention
This invention pertains generally to refrigeration systems, and more specifically to removable intake screens for air flow panels used in tractor-trailer type vehicular refrigeration units. The air flow panels distribute air flow throughout the trailer, so a refrigeration system may maintain sensitive cargo at a controlled temperature.
2. Description of the Related Art
Cargo is transported throughout the world in transportation containers. Over moderate land distances, these containers are most frequently trailers pulled by large truck tractors. Tractor trailers offer flexible distribution of cargo to nearly any location, and relatively rapid loading and unloading of cargo. The cargo is often grouped onto pallets, and then loaded with pallet jacks or fork lifts onto the trailers. Since the pallets are generally loaded at a warehouse or distribution center, diverse cargo may be loaded and unloaded quickly and reliably, and then transported by a driver to one or more destinations. Except in special circumstances such as livestock trailers, these trailers are closed and sealed to prevent roadway dirt, moisture and other contamination from damaging the load.
Unfortunately, during transport these trailers may reach temperature extremes that can also be damaging to some types of cargo. To protect more sensitive cargo, some trailers are equipped with refrigeration units. The refrigeration units are commonly mounted onto the front wall of the trailer where they are out of the way of cargo loading and unloading. The refrigeration units are designed with sufficient capacity to maintain constant temperature within a trailer, and generally have very large air volume throughput capacities. In conjunction with these refrigeration units, the load may be arranged to provide appropriate air passageways around and through, so that an even temperature is sustained throughout the entire load. Alternatively, the cargo may be divided into temperature controlled chambers and chambers without special temperature control, or into sections of various temperatures. A single trailer may then be used to carry temperature insensitive contents such as paper products together on the same load with produce, flowers or frozen goods.
While most refrigeration units are capable of moving very large volumes of air, maintaining that air flow throughout the cargo area has proven to be much more difficult. Where load arrangement is the only source of air flow control, protection of the cargo has been unreliable during transit, since the cargo tends to shift in response to sudden changes in direction or velocity. A sudden stop may shift the load towards the front wall of the trailer, which may then block the air inlet or outlet of the refrigeration unit. Furthermore, different fork lift operators and other loading personnel each may arrange a load differently. In some instances, the forklift operator may accidentally position cargo too close to the refrigeration unit. Variable load arrangement, either at the time of loading or during transit, has resulted in loss of many loads of cargo.
A number of attempts have been made in the prior art to reduce or eliminate these unfortunate losses of cargo, while simultaneously simplifying the job of the loading personnel. One early attempt involved the placement of a lattice panel over the front of the trailer. The lattice panel forced the cargo away from the front wall, thereby ensuring an air passageway between the load and front wall. While this lattice panel prevented blockage of the refrigeration unit intake and outlet, an improper load arrangement could still block the flow of air to the back of the trailer. In such an instance, outlet air would be drawn directly back to the inlet of the refrigeration unit without first passing over the cargo. The refrigeration unit would detect the appropriate environment temperature and shut off prematurely, still resulting in potential cargo spoilage or damage.
Solid bulkheads were developed to direct air through passageways along the exterior wall of the trailer. The bulkheads help ensure proper passage and distribution of air by separating the air from the load until the air is distributed to or drawn from appropriate places within the trailer. Early intake bulkheads were simply solid panels placed over the lattice work that had intake openings at the bottom of the trailer. An excellent example of a more recent corrugated bulkhead is disclosed by Severson in U.S. Pat. No. 4,399,737, the contents which are incorporated herein by reference. The corrugations disclosed therein provide structural strength in a unitary panel without requiring the addition of a large quantity of lattice. Nevertheless, with the advent of these solid bulkheads came other problems. For example, plastic, paper and other debris often drops to the bottom of the trailer during loading and unloading. This debris was inconsequential when the refrigeration unit was located high on the front trailer wall. The Severson type bulkheads and ducts, however, draw air from the floor and, in the process, also draw in the debris. The debris accumulates in either the bulkhead or the refrigeration unit, eventually reducing the air flow through the refrigeration unit to an unacceptable level. Structural integrity is also of great consequence in the tractor trailer environment, since the sudden stops that may cause load shifting may also on occasion launch the cargo into the unitary panel. The panel must be of sufficient strength and integrity to withstand the occasional high force impact and not crack, break or otherwise fail.
To prevent the debris from blocking flow through the inlet bulkhead, other variations have provided screens. Some of these place the screens at the air intake along the bottom. Unfortunately, these tend to quickly draw the debris from the floor, where the debris naturally rests, so the intake screens tend to become quickly clogged. Once the intake screen clogs, the refrigeration unit air flow reduces below a minimum acceptable level and the refrigeration unit will shut down. Again, this results in damaged cargo.
Other versions have raised the screen off of the floor, which tends to reduce the amount of debris which gets drawn in. Furthermore, when the refrigeration unit is shut off, the debris will drop from the screen down to the floor. Unfortunately, in order to move the intake screens up off of the floor, the screens have been located internally of the bulkhead and have been permanently attached, such as with rivets. While much of the debris will readily fall from the screen, there is always a certain content of the debris that for one reason or another will not detach. For example, many of the packages have adhesives that remain somewhat tacky. Other times, when a package is accidentally destroyed and contains sticky contents, such as ice cream or other confectionery, these packages or the debris therefrom will remain adhered to the screen even after the refrigerator unit shuts off. Because the screen is behind the bulkhead and is permanently attached, cleaning is extremely difficult.
More recently, Onken, in U.S. Pat. No. 5,807,046 also incorporated herein by reference, discloses a solid bulkhead having a number of flutes, or ridges, formed in the bulkhead wall. Air flow through the bulkhead may then be tailored by selectively opening or closing various ones of the flutes. However, no provision is made for debris that might get drawn into the flutes. As with earlier versions, this debris will accumulate within the bulkhead and eventually restrict air flow. Cleaning the bulkhead is an extremely difficult task.
Henning et al, in U.S. Pat. No. 5,769,704, the contents which are incorporated herein by reference, also disclose a unitary panel type bulkhead, but rely solely upon several sinusoidal air vanes therein for support. The base of the Henning et al structure which provides air inlet is relatively unsupported, and may be readily destroyed when impacted by cargo or forklift tines, which might extend out at the base of a load of cargo.